1. Field of the Invention
The invention relates to a motor vehicle brake system having a hydraulic line.
2. Description of the Related Art
The invention relates to a motor vehicle brake system having a hydraulic line via which brake pressure can be applied by a brake cylinder to a wheel brake module by means of a braking medium, and to which a low pressure accumulator for temporarily receiving excess braking medium is connected, wherein the low pressure accumulator is connected to the hydraulic line in order to feed back buffered braking medium via a feedback line and a feedback pump which is connected into the feedback line, and the feedback pump is actuated cyclically in order to set the delivery capacity in such a way that activation occurs within each braking cycle during a portion of pump strokes corresponding to the delivery capacity, and to a method for its operation.
Such a brake system is generally used within the scope of an ABS system, wherein, in such systems, the feed pump which is connected into the feedback line is typically controlled as a function of what is referred to as the admission pressure, that is to say the pressure which is present between the master brake cylinder and the inlet valve of the wheel brake module.
DE 199 46 777 A1 discloses a method which uses the run-on voltage of a motor of a pump which is operated in a clocked fashion and which is used to feed back brake fluid from an accumulator chamber, arranged at the outlet end of a wheel brake cylinder, into a master brake cylinder, and the accumulator chamber pressure which is determined or measured in the accumulator chamber, to estimate an admission pressure prevailing between the master brake cylinder and an inlet valve of the wheel brake cylinder of a motor vehicle brake system.
Furthermore, DE 10 2005 041 556 A1 discloses a method for determining the admission pressure which prevails between the master brake cylinder and the inlet valve of the wheel brake cylinder. The latter determines the admission pressure taking into account the profile of a run-on voltage of a motor of a pump which is operated in a clocked fashion and which is used to feed back brake fluid from a low pressure accumulator into the master brake cylinder. In this context, a plurality of characteristic variables of the voltage profile are measured and are respectively used to determine an admission pressure value. Furthermore, the admission pressure values which are determined from the various characteristic variables are averaged, and the admission pressure values are averaged over time in order to damp fluctuations.
To do this, use is made of evaluation of the quality and/or reliability of the measured characteristic variables, filtering and/or conditioning of the characteristic variables and/or of the admission pressure values determined therefrom when there is insufficient quality and/or reliability of the measured characteristic variables, and only pressure values of a similar magnitude are used for this.
The motor vehicle brake system can comprise one or else a plurality of low pressure accumulators. Two low pressure accumulators, which do not necessarily have to have the same filling level during operation, are preferably provided.
The filling level of the low pressure accumulator results from the volume which is let into the low pressure accumulator by the outlet valves for the wheel cylinders, and from the feed volume of the pump which empties the low pressure accumulator. The inlet quantity during an ABS control process is known to depend on the actuation time of the outlet valves and the wheel cylinder pressure. The wheel cylinder pressure is known to be able to be calculated on a model basis if no pressure sensor is present, and it is usually available in an ABS control program as a parameter. The actuation times are also known so that the volume inflow can also be determined by approximation. The volume inflow from the low pressure accumulator is obtained in a way known per se by considering the generator voltage of the pump. Finally, it is therefore possible to calculate, from the available information, the filling level of the low pressure accumulator during an ABS control process at least to a certain degree of approximation (low pressure accumulator model).
If a wheel in an ABS-controlled brake system exhibits the tendency to come to a standstill, the brake pressure in the hydraulic brake circuit is lowered by virtue of the fact that the quantity of brake fluid is reduced in the circuit. This extracted brake fluid is buffered in a number of separate low pressure accumulators and can easily be fed back again into the brake circuit when a corresponding request occurs.
The available quantity of brake fluid which is measured or estimated on the basis of the aforesaid model calculation is used in the brake system of a motor vehicle as main criterion for assessment of a situation in the LPA. The behavior of the wheels of the vehicle is also evaluated for the sake of completeness and for self-monitoring of the vehicle systems.
The entire ABS-controlled braking process will only require minimum switching-on of the brake pump. The following requirements are made of the brake system for this purpose:    1. The pump is to be used as economically as possible.    2. The noise of the pump is to be reduced as far as possible.    3. As a function of the pump activity it should be ensured that only relatively small fluctuations occur in the rise in pressure.    4. The rebound of the brake pedal will be reduced as far as possible.    5. At any time, sufficient free space is to be available in the LPR or LPA to take up excess pressure.    6. All the said requirements are to be capable of being met independently of the type of vehicle and of operating temperatures and of other conditions.
During an ABS-controlled braking process, the quantity of brake fluid within the LPA is the result of two processes running in parallel:    1. Inflow (Vin): the quantity of brake fluid which is extracted from the brake circuit characterizes a drop in pressure which is caused by the ABS system.    2. Outflow (Vout): this is understood to be the return flow of brake fluid into what is referred to as the THC circuit. THC is the term for what is referred to as the tandem master cylinder, i.e. the master brake cylinder which is used to take up the brake fluid and to increase pressure on request.
As a result, the quantity of brake fluid which is available in the LPA is calculated from the formula VLPA=Vin−Vout.
A low pressure accumulator is known to have a pressure profile which is dependent on the filling level. If the filling level is low, the pressure in the low pressure accumulator is low. The higher the filling level, the higher the pressure in the low pressure accumulator. The feedback pump which is present in a motor vehicle brake system with a low pressure accumulator could in principle always be operated at full power, with the result that the low pressure accumulator would be continuously empty. However, for reasons of energy consumption, of the generation of noise and of the pedal sensation, this is not expedient, wherein in particular an excessively frequent pump activity reduces the pedal sensation.
In addition, methods which are known per se for controlling the pumps, for example control by means of which the wheel pressure is adjusted from an actual value to a predefined setpoint value, are not optimal in all driving situations. For example, when brakes are adjusted to a very low coefficient of friction, the low pressure accumulator may not be sufficiently emptied.
In this case, the pressure in the low pressure accumulator is too large to permit the brakes to be released in accordance with the demand by allowing the braking medium to flow out of the hydraulic line via the outlet valves of the wheel cylinders into the low pressure accumulator, and therefore permit a reduction in the pressure in the wheel brake module. The operational reliability of the system could therefore be adversely affected.